‘Superstar’ Eta Carinae acts as a giant cosmic-ray gun, but why?

10 July 2018

4 min read

The star Eta Carinae erupted in the 1840s, as seen from Earth, the creation of the Homunculus Nebula are visible in this photo from the Hubble Space Telescope. New work shows the system spitting out ultra-fast ‘cosmic radiation’. Credit: NASA, ESA, the Hubble SM4 ERO Team

A big star 7500 light-years away is shooting out radiation at superhigh speeds. A new study using NASA’s nustar space telescope show that Eta Carinae may act as an accelerator of charged particles, or cosmic rays.

Eta Carinae is a well-known hourglass-shaped cloud of gas with two massive stars orbiting each other: They are 30 and 90 times the mass of the sun, respectively. The system experienced an eruption in the 19th century, and briefly became the second brightest object in the sky. Astronomers, however, still struggle with the understanding of the source of the explosion, and the dynamics of the star-system in general.

“We know that the blast waves from exploded stars, to be able to accelerate cosmic particles to speeds comparable with that of light, an incredible energy boost,” lead author Kenji Hamaguchi, an astrophysicist at NASA’s Goddard Space Flight Center in Maryland, said in a statement from NASA. “Similar processes must occur in other extreme environments. Our analysis indicates that Eta Carinae is one of them.” [Eta Carinae: An Explosive Star System HD Images]

Cosmic rays are high-energy particles, such as electrons, protons, and atomic nuclei originating outside the solar system. Astronomers do not know for sure where cosmic rays come from, researchers said in the statement, because these particles carry a burden. This means that the particles’ paths through space, change and scramble when the particles meet the magnetic field.

However, variations in Eta Carinae radiation output showed that something interesting was happening in that system, leading researchers suspect that it could be one of these elusive cosmic-ray sources.

“Both of Eta Carinae stars drive powerful outflows called stellar winds,” said team member Michael Corcoran, senior research scientist at Goddard. He said: where the wind crosses, there are periodic changes in the “soft” or low-energy X-rays. These changes have been observed for two decades.

NASA’s Fermi Gamma-ray Space Telescope also saw changes in gamma-rays (a more energetic form of radiation than X-rays) in the direction of Eta Carinae. However, Fermi is a bit myopic than X-ray telescopes, making it difficult to figure out if the gamma-rays are originated from Eta Carinae.

So nustar took a look. The team used archival data (as well as new observations) of the nustar between March 2014 and June 2016. During the same period, the members of the team also collected lower-energy X-ray observations from the European space agency’s XMM-Newton satellite.

Nustar can investigate high-energy X-rays which are emitting radiation above 30,000 electron volts (eV), visible light, by contrast, only has an energy of about 2 eV to 3 eV. The telescope observations showed that the variations in these “hard” X-rays have a similar pattern with the gamma-ray fluctuations, the Fermi telescope observed.

“The researchers say that the best explanation for both the hard X-ray and gamma-ray emission is electrons accelerated in violent shock waves along the boundary of the colliding stellar winds,” NASA officials said. “The X-rays detected by nustar and gamma radiation detected by Fermi emergence of starlight gets a huge boost of energy by the interaction with these electrons.”

NASA added that a number of these ultra-fast electrons, as well as other charged particles, probably to escape Eta Carinae. Some of these particles can fly in the Earth direction, the result here as a cosmic-ray detections.

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